Three forms of reductionism are currently being advocated seriously. (1) The doctrine of constitutive (or «ontological») reductionism, according to which no biological phenomenon conflicts with physico-chemical explanations, is now generally accepted. (2) According to theoretical reductionism, which is the most recent expression of the logical-empiricist doctrine of the unity of science, the theories formulated in biology are special cases of physico-chemical theories. (3) According to explanatory reductionism, all biological phenomena can be explained in terms of the actions and interactions of their components. The first form of reductionism turns out to be too weak. The second form, which is closely related to Hempel's deductive-nomological explanation scheme, is too strong to characterize research in biology. However, models of reductive explanation — in particular, Wimsatt's model of explanatory reduction and Kauffman's model of explanation by articulation of parts — reconstruct scientific practices in biology more adequately. In the context of explanatory reductionism, various forms of emergence can be defined which (contrary to current opinion about emergence) do not at all exclude reducibility.